Posts Tagged ‘S-Fold’

After leaving the Garth Run outcrop we drove collectively in our vans (one being far superior to the others) to an area along Route 33 just near the entrance to Skyline Drive. Here we pulled over, and piled out of the vans and into the woods. There was no trail for us to follow here, just a creek bed and an intrepid guide/professor. But, even in the rough overgrowth each member of the excursion made sure to mind their footing and the branches of troublesome trees… well, some of us anyways (not naming names). Even so, all of us were troopers and muscled on motivated by the promise of pizza.

At the outcrop itself we saw sedimentary rocks instead of igneous/metamorphic this time. The Swift Run Formation (Google Earth File) is comprised of arkose sandstone and some finer sedimentary rocks including mud rip-up clasts that were not easily recognizable at first glance. Originally thought to be some type of xenoliths, these mud clasts were put in place by a large increase in depositional energy that tore these mud chips from the bottom and then displaced them amongst the sandstone. Further along the outcrop there was a rock that exhibited the transition from coarser to finer grain sizes drastically.

There was also some interesting tiger-stripe-like foliation seen.

There was also evidence for tectonic effects seen in some folding. The first fold found was an overturned syncline that was only about 10 cm in length and found at the base of the outcrop. It was noticeable because the darker mud layers contrast nicely against the lighter grained arkose. Interesting about this fold and the second set found were smaller, in this case parasitic, S-folds found. In both cases the s-fold has an East over West, or sinistral orientation. This is because the orogeny was coming from the east, and would have pushed the bedding layers in such a direction.

The Swift Run Formation was deposited during rifting of the super continent Rodinia. The sediments would have been deposited in an environment similar to modern day the Culpeper Basin. It overlays the basement complex seen at the first outcrop, but is underneath the Catoctin basalts that were deposited over top as rifting of the continent progressed. The folding witnessed here would be attributed to the Alleghenian Orogeny, which will be a common theme through the next couple of posts.

After about an hour of pressing our faces up against the rock we made the trek back through the brush to the vans.  We pressed onward to find the elusive 10-topping pizza.


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